1. Field of the Invention
This invention relates to biocompatible coatings for orthopedic implants. The invention coating shields the implant material from corrosive bodily fluids and prevents the gradual leaching or ionization of the implant substrate material into surrounding body tissue.
2. Background of the Invention
Orthopedic implants are currently manufactured from a variety of materials, most commonly metals, metal alloys and ceramics. The use of polymers and composites is still in its infancy but is expected to increase. Apart from the mechanical strength and modulus requirements that an implant material must possess to provide useful implants with reasonable life spans, the implant material must also be compatible with body tissue. Further, the implant material should be resistant to the corrosive or leaching action of body fluids.
Substrate materials such as metals and their alloys ( collectively referred to hereafter as "metals") possess the necessary strength for use as orthopedic implants. However, metals generally have a higher modulus of elasticity than bone. Consequently, when metallic implants are used in a load-bearing function, the load is not always effectively transferred to the bone. This could result in gradual bone resorption, which can have serious effects, especially in the case of young implant recipients. Over a period of time, the degree of bone resorption could result in loosening of the implant necessitating surgery to perform remedial action such as a replacement prosthesis.
Metals are not completely inert in the body but are generally prone to corrosion by ionization in body fluids. This ionization can be acute when a metallic prosthesis articulates against an opposing surface. The articulating action repeatedly clears the surface of any passive oxide film that might develop on the surface and continually exposes fresh metallic surface to the body fluids.
It has been suggested that metallic implants can be coated to improve their tissue compatibility by reducing or preventing ionization corrosion. Thus, for instance, U.S. Pat. No. 4,145,764 to Suzuki et al. recognized that while metal prostheses have excellent mechanical strength, they tend to corrode in the body by ionization. To solve this problem, Suzuki proposed a metal prosthesis plasma sprayed with a bonding agent which is in turn covered with a porous ceramic coating which would allow the ingrowth of bone spicules into the pores. This combination, Suzuki claimed, would provide both the mechanical strength of metals and the biocompatibility of ceramics.
Suzuki did not, however, address the issue of dimensional changes that occur when applying a coating or the effect of these dimensional changes in the tightness of fit between the surfaces of an articulating joint prosthesis. Further, the application of ceramic coatings to metal substrates often results in non-uniform, poorly-bonded coatings which tend to crack due to differences in thermal expansion between the ceramic and the underlying metal substrate. Furthermore, such coatings are relatively thick (50-300 microns) and since the bond between the metal and the ceramic coating is often weak, there is always the risk of galling or separation of the ceramic coating.
U.S. Pat. No. 4,164,794 to Spector et al. relates to prosthetic devices fabricated from or coated with selected thermoplastics referred to as "bioengineering thermoplastics." The prostheses have an inner load bearing portion and an outer sintered or foamed porous coating of a bioengineering thermoplastic. The coatings are 0.5 to 10 mm thick and the average pore diameter is 90-600 microns. The function of the pores is apparently to allow ingrowth of bone spicules to stabilize the implant in the body, with no recognition that there could be any benefit in preventing ionization of substrate materials.
Polymeric prostheses with substrates formed of thermoplastics such as high density polyethylene (HDPE) have been tried, but are not as biocompatible as originally assumed according to U.S. Pat. No. 4,356,571. According to this it has been found that over a long period of time a layer of connective tissue can form on the HDPE surface. This layer may be such as to cause loosening of the implant so that reimplantation or other remedial action becomes necessary. Further, according to the '571 patent, it has been found that thermoplastics generally are not as passive to body fluids as originally thought. Body fluids can invade the thermoplastic composition by a leaching or diffusion action resulting in a weakening of the implant. Further, the leaching action releases the implant composition into the body in small quantities, with unknown long term effects.
Further, according to U.S. Pat. No. 4,356,571, it was generally believed that thermosetting resins could not be used as implant materials because they would always release substances that were not biocompatible. The Esper patent is directed to prosthetic implants produced entirely of cured fiber reinforced composites of a biocompatible, thermosetting "triazine resin."
Recent studies suggest that carbon fiber composites may exhibit significantly higher electrochemical activity than metal alloys due to the high electrical conductivity of carbon fibers and the absence of an electrically resistant oxide layer such as those spontaneously formed over metal alloys. While clinical effects of this conductivity are not yet known, it has been suggested that these electrical interactions are undesirable.
In view of the unique advantages of each of the many different types of materials in the fabrication and use of prosthetic devices, there exists a need for a means of coating an implant formed, for example, of thermoplastic, metallic or composite materials to render it inert in the body. Such a coating must be of a type that would render a metallic or composite implant immune to corrosion by ionization and a thermoplastic, metallic or composite implant immune to corrosion, leaching and invasion by bodily fluids.